MIMO is one of the most important technologies for realizing a high data rate required in a radio data system. Data streams can be transmitted via MIMO so as to improve the system throughput. Currently, MIMO is applied to most 3G and 4G radio standards, e.g. World interoperability for Microwave Access (WiMAX) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) and Long Term Evolution (LTE) technology.
The transmission schemes of the MIMO system are mainly classified as two types: open-loop MIMO system and closed-loop MIMO system. A closed-loop MIMO system feeds channel information at a receiver back to a transmitter and then performs operations such as pre-coding and beam-forming on the transmission data. But reciprocating communication is not performed by a receiver and a transmitter in an open-loop MIMO system, thus channel information cannot be fully utilized. In a scenario that a channel changes slowly, a closed-loop pre-coding MIMO system needs to improve the system performance by using the fed-back channel information. Since a feedback link occupies the system overhead, in a practical system, a partial feedback technology is used to allocate a limited feedback channel between a mobile phone and a base station to feed back important information of the channel and realize multiplexing technology. In order to reduce the signaling overhead of a reverse link and the fed-back information of a pre-coding system, an LTE system adopts a codebook-based pre-coding technology to improve the spectral efficiency of a system. For an LTE closed-loop MIMO system, a pre-coding matrix and a corresponding index value are specified in a protocol. A receiver selects an optimal pre-coding matrix according to a certain criterion and feeds a Precoding Matrix Indicator (PMI) back to a transmitter which then obtains the pre-coding matrix according to the PMI and performs pre-coding on transmission signals.
Criteria for estimating PMI in the existing closed-loop MIMO system include the Minimum Mean Squared Error (MMSE) matrix trace criterion, the maximum channel capacity criterion and the optimal Bit Error Rate (BER) selection criterion etc.
In a non-singular case, the gain of a closed loop is not large than that of an open loop according to the MMSE matrix trace criterion; a calculated theoretical channel capacity can not be actually achieved and there is no gain in the Frame Error Rate (FER) performance according to the maximum channel capacity criterion; while the optimal BER selection criterion is equivalent to the maximum throughput criterion and has a shortcoming that its calculation formula is obtained on the premise of a large number of assumptions. Therefore, the existing pre-coding matrix selecting methods have some disadvantages.